1. Field of the Invention
The present invention relates to an image forming apparatus including: a fixing unit configured to form a nip portion between a flexible endless belt and a press member and perform fixing on a recording medium passing through the nip portion.
2. Description of the Related Art
An electrophotographic image forming apparatus forms a latent electrostatic image on the surface of a photoconductor drum serving as an image bearing member and develops the latent electrostatic image on the photoconductor drum with a developer such as an electrostatic image developing toner to form a visible toner image. The electrophotographic image forming apparatus is configured such that the toner image is transferred onto recording paper with a transfer device, and heat and pressure are applied with a fixing device to the recording paper onto which the toner image has been transferred, whereby the toner image on the recording paper is fixed and then the recording paper is discharged outside of the apparatus.
In general, the fixing device has a pair of a heat roller and a press roller facing each other, or a fixing belt, or a combination thereof. The fixing device holds a conveyed recording paper sheet at a nip between the heat roller and the press roller, and applies heat and pressure to the recoding paper sheet passing through the nip to thereby fix the toner image thereon. The fixing device described in Japanese Patent Application Laid-Open (JP-A) No. 2004-286922 is known as such a fixing device that employs a belt.
This belt fixing device described in JP-A No. 2004-286922 has: a nip-forming member inside a fixing belt in the form of an endless sheet to be heated; and a rotatable press roller which is pressed against the nip-forming member via the fixing belt, wherein the contact portion of the fixing belt with the press roller is a fixing nip. Inside the fixing belt is provided a heater lamp for heating the fixing belt which is disposed at a position distant from the fixing nip, and the fixing nip is located above the heater lamp. In addition, a thermistor is provided at a part of the fixing belt. By controlling the heater lamp to be on or off on the basis of the temperature detected by the thermistor, the heat roller and the fixing belt can be adjusted to desired temperatures.
Also, the fixing device described in Japanese Patent (JP-B) No. 2861280 using a heating device has a heating body and a driving member configured to form a nip with the heating body via a slidably moving film and to drive the film. This fixing device applies heat from the heating body via the film to a recording medium carrying an image at the nip to thereby fix the image thereon. In addition, the fixing device described in JP-B No. 3472371 using a heating device is the same as that of JP-B No. 2861280 except that the heating portion for film fixing of the heating device is replaced with a heater and a nip-forming member. Furthermore, as a fixing device having a simple configuration, there has also been proposed one having a fixing belt serving as a heating member, a press roller serving as a press member, a support member, a heat source and a backing member.
In image forming apparatuses based on an electrophotographic or electrostatic recording method containing the fixing devices described in JP-A No. 2004-286922 and JP-B Nos. 2861280 and 3472371, a toner used is deposited at a developing step on an image bearing member (e.g., a photoconductor) having an electrostatic image thereon. Next, the toner image formed on the photoconductor is transferred from the photoconductor onto a transfer medium such as transfer paper at a transfer step, and then fixed on the transfer medium at a fixing step.
JP-A No. 2007-233011 studied improvements in electrostatic image developing toner in order to ensure glossiness of a full-color image while preventing offset which is one problem in a fixing step.
Amid recently increased concerns about environmental protection, energy saving has strongly been required for the above image forming apparatuses. Specifically, they have been required that the power consumption during fixing should be reduced to the greatest extent possible. Meanwhile, it is necessary to obtain good fixing performance even at low fixing temperatures. At present, components of image forming apparatuses have become simple possible because of their reduction in size and weight. Therefore, it is inevitable to further improve toners in their properties in order for image forming apparatuses to be excellent in low-temperature fixing property without a complex configuration and to form high-quality images.
One common method for fixing a toner image on a transfer medium such as transfer paper is a heat-press method. This method is a method where a toner image is fixed by moving an object having the toner image while the toner image is being brought into contact under pressure with a surface of a fixing member which surface is made of a material having releaseability from the toner. Since the surface of the fixing member is in contact with the toner image of the transfer target under pressure, this heat-press method is remarkably excellent in heat conversion efficiency and can perform fixing rapidly. However, in this method, the heat roller of the fixing member is in contact under pressure with the toner image having a heated and molten state and thus when the temperature of the heating roller is too high, the toner causes hot offset which is a problem that the toner melts excessively and adheres to the heat roller. Meanwhile, when the temperature of the heat roller is too law, the toner does not melt sufficiently, leading to gradation in temperature inside the toner layer. As a result, the boundary temperature between the lowermost surface of the toner layer and the surface of the medium does not reach a temperature enough to melt the toner, so that the toner layer is broken, causing cold offset which is a problem that fixing becomes insufficient. Therefore, demand has arisen for development of a toner which is excellent in offset resistance and realizes low-temperature fixing property.
Fixing at low temperatures can be achieved by reducing the molecular weight and/or the glass transition temperature of a binder resin to reduce the softening temperature of the toner; i.e., reduce the lowest fixing temperature of the toner. However, reducing the molecular weight and/or the glass transition temperature of a binder resin makes it easier for the obtained toner to aggregate, so that the toner is degraded in storage stability and causes problems such as blocking. Also, an attempt to improve offset resistance by increasing the elasticity of a binder resin by, for example, increasing its molecular weight tends to increase the lowest fixing temperature of the toner conversely.
In order to solve these problems, there have been proposed attempts to attain both desired low-temperature fixing property and desired offset resistance by focusing on rheology properties of a toner and control viscoelastic behaviors thereof (e.g., JP-A Nos. 04-353866 and 09-006051). The electrophotographic toner of JP-A No. 04-353866 has such rheology properties that a temperature at which its storage modulus starts to decrease falls within 100° C. to 110° C., a storage modulus G′(150) at 150° C. is 1×104 dyn/cm2 or lower, and the peak temperature of the loss modulus G′ is 125° C. or higher. The storage modulus G′ and the loss modulus G″ are respectively the real part and the imaginary part of a complex modulus, which is one of characteristic functions of viscoelasticity defined in common vibration experiments of objects having viscoelasticity. Specifically, the storage modulus G′ indicates the elasticity of the toner and the loss modulus G″ indicates the viscosity of the toner. However, the electrophotographic toner of JP-A No. 04-353866 has a high storage modulus G′(150) at 150° C. and a high peak temperature of the loss modulus, and thus is poor in low-temperature fixing property. The electrophotographic image developing toner of JP-A No. 09-006051 contains a binder resin whose storage modulus G′ at 120° C. falls within a specific range and complex modulus at 100° C. falls within a specific range. However, even the electrophotographic image developing toner of JP-A No. 09-006051 has too high a storage modulus G′(120) at 120° C. and thus is poor in low-temperature fixing property.
In addition, in order to obtain an excellent image as a color print, a toner good in color mixing property and color developing property is required. In particular, irregularities on the surface of the fixed image reflect light diffusively, so that the glossiness is degraded and the color developing property is also degraded. Thus, it is desirable to form a fixed image having a moderate glossiness and changing in glossiness to a lesser extent even when the fixing temperature is changed.
The toner described in JP-A No. 08-179563 is known as an electrophotgraphic color toner controlled in viscoelastic behaviors to attain stable, high glossiness. This electrophotgraphic color toner of JP-A No. 08-179563 satisfies the expression: 1.2≦log(5.0×104)−log A≦1.8, where A is a storage modulus G′ [dyn/cm2] at T1+20 [° C.], where T1 is a temperature [° C.] when a storage modulus G′ measured under vibration at a frequency of 1,000 Hz is 5×104 dyn/cm2.
In recent years, the belt fixing devices as described in JP-A No. 2004-286922 have been required to further be shortened in warm-up time (i.e., the time required that the device reaches from a normal temperature to a predetermined temperature (reload temperature) at which printing can be performed after its power source is on) and first print output time (i.e., the time required that a series of printing steps is completed consisting of receiving a printing order, performing a pre-printing step, performing printing operations and discharging paper sheets). As the process speed of image forming apparatuses increases, the number of paper sheets fed per unit time increases to increase a necessary amount of heat. As a result, an amount of heat is insufficient especially at the beginning of continuous printing (i.e., a drop in temperature), which is problematic.
The film fixation of JP-B No. 2861280 is effective as a method for solving the problem in JP-A No. 2004-286922. This method enables a fixing device to be reduced in heat capacity and size as compared with the belt fixing device of JP-A No. 2004-286922. This fixing device locally heats its nip portion only, and the other portions are not heated. As a result, the belt is the coldest at a portion of the nip which a paper sheet enters, which tend to cause fixing failures. Particularly in high-speed devices, more severe fixing failures tend to occur since the rotation speed of the belt is high and thus a more amount of heat is released from the belt at the other portions than the nip.
In the fixing method using a belt (or a film), the belt is moved in a thrust direction while rotated and goes beyond a thrust-preventing member or hits it to buckle, so that the belt may be broken. In some cases, when paper jam at a fixing portion is reversed, force is locally applied to the belt to fold it, so that a fine mark of kink remains. Such a mark may break the belt. A large mark of kink is reflected on the obtained image, causing image failures.
In SURF fixing, a belt has therein a resin holder for retaining a ceramic heater and a metal stay for supporting the belt so as not to bend. This configuration has a problem that the heat capacity inside the belt is so large that heat is absorbed by these members and thus heat conversion efficiency is low (which is an obstacle of energy saving). As seen in SURF fixing, when the belt is rotated distant from a heating body (exothermic body), the belt is cooled due to forced convection, so that heat loss increases to degrade heat conversion efficiency.
The fixing device of JP-B No. 3472371 has a guide roller for stretching a belt from inside of the belt. Such a member in contact with the belt absorbs the heat from the belt to extend the warm-up time and the first print output time. Also, when the nip portion is the parallelism between and the guide roller is lost only slightly, declination occurs between the right-hand portion and left-hand portion in an axial direction of the belt in resistance of rotation of the belt, causing a so-called belt bias.
JP-A No. 2007-233011 studied improvements in toner in order to ensure glossiness of a full-color image while preventing offset which is one problem in a fixing step. In order to obtain a full-color image, it is necessary to make a surface of an image smooth and uniform to ensure glossiness. In addition, it is necessary to prevent expansion of the color-reproducible range due to melting between toner particles and occurrence of the above-described cold offset in a region where the image density is high. Thus, a sufficient amount of heat is applied to the toner for fixing in order to prevent cold offset from occurring. Effective methods for applying a sufficient amount of heat are: increasing the temperature of the heat roller; and reducing the hardness of rubber of the press roller and increasing the diameters of the heat roller and the press roller to expand the nip portion and extend the nip time. Meanwhile, when a sufficient amount of heat is applied to the toner, the toner decreases in elasticity and easier to cause the above-described hot offset. The hot offset can effectively be prevented by coating the heat roller surface with silicone oil which makes it increase in releaseability. This measure is not completely effective and leads to shortening the service life of the apparatus and enlarging it. Thus, it is not a clever measure. Also, silicone oil used for increasing releaseability is desirably reduced to the greatest extent possible since it contaminates the interior of the apparatus during double-side printing, and it is more difficult to write letters or something on the printed matter.
The electrophotographic toner of JP-A No. 04-353866 has a high storage modulus G′(150) at 150° C. and a high peak temperature of the loss modulus, and thus is poor in low-temperature fixing property. Also, even the electrophotographic image developing toner of JP-A No. 09-006051 has too high a storage modulus G′(120) at 120° C. and thus is poor in low-temperature fixing property.
The electrophotographic color toner of JP-A No. 08-179563 has too low a storage modulus G′ at T1+20 [° C.] which is A satisfying the above expression, and thus is poor in offset resistance. In general, when the storage modulus G′ is lower, the viscosity of the toner becomes predominant. Such a toner tends to move onto the fixing member, and is poor in offset resistance. In contrast, when the storage modulus G′ is higher, the toner is restored in shape after fixing by its elasticity, so that irregularities are easier to form on its surface and glossiness is lost. As described above, it is a quite difficult problem to attain both desired offset resistance and desired glossiness.
The present invention has been made in view of the above-described problems, and aims to an image forming apparatus containing in combination a fixing unit and an electrostatic image developing toner, which can shorten the first print output time from the waiting state, which can overcome shortage of heat during high-speed rotation, which can attain excellent fixing property when mounted in mass-productive image forming apparatuses, and which can exhibit both good offset resistance and good glossiness at low fixing temperatures.